Analysis of Various Steel Bracing Systems using Steel Sections for High Rise Structures Y. U. Kulkarni 1 P. G. Chandak 1 M. K. Devtale 1 S.S. Sayyed 1 1 Assistant Professor, Department of Civil Engineering, Annasaheb Dange College of Engineering & Technology, Ashta. Abstract Steel structures can be strengthened in various types, to resist the lateral forces coming on the structure. In steel buildings, the most widely used method of constructing lateral load resisting system is using braced frames. These systems help the structure to reduce the buckling of columns and beams and the stiffness of the structure is increased. Hence, the main concern while analyzing steel structure is to select the appropriate bracing model and to decide the suitable connection type. The steel bracings are also used in retrofitting to give maximum strength to the structure. In this study, the behavior of the bracing system is analyze for parameters like displacement, weight etc by using different types of bracing systems e.g. X bracing, V bracing, Inverted V bracings, Knee bracings. These bracings are analyzed and designed by using STAAD Pro V8i software. For this analysis the G+9 storey building with different bracing system has been analyzed and designed as per Indian Standard Code 800-2007 by using UC and UB (British) sections for the wind, earthquake and gravity loads also their combinations. Keywords - Bracing System, X bracing, V bracing, Inverted V bracings, Knee bracings. 1. Introduction: The High rise steel structures are now a day s very popular because of the ease of erection and high strength-weight ratio. The stability of the high rise structures from the lateral loads like wind and earthquake is important. The structure can be stabilizing by using the bracing system, moment resisting connections, and also a shear wall. But the mostly favorable type to resist these lateral loads is bracing system in steel structure. The suitability of the bracing system is depends upon the behavior of the bracing to the lateral loads act on the structure. The IS: 1893 is used for the earthquake load calculation. For wind load calculation IS 875: 1984 is used. The structure is designed by using IS 800:2007 for Limit State method of design. 2. Steel Bracing System: Bracing is a highly efficient and economical method to laterally stiffen the frame structures against wind and other lateral loads. A braced bent consists of usual columns and girders whose primary purpose is to support the gravity loading, and diagonal bracing members that are connected so that total set of members forms a vertical cantilever truss to resist the horizontal forces. Bracing is efficient because the diagonals work in axial stress and therefore call for minimum member sizes in providing the stiffness and strength against horizontal shear. 2.1 Types of Bracings 2.1.1 Concentrically Braced Frames (CBFs) concentrically braced frames (CBFs) (Fig.1) having simple connections, it is assumed that the centroidal axes of the members meet at a common point at each joint and so the members carry essentially axial loads. 2.1.1.1 X-Bracing This is the most economical and efficient forms of bracing. When the cross bracing (Fig.1 (a)) is used, lateral force from one direction induces tension in one member while the other brace is in tension when the force is reversed. Therefore, if two diagonals are used, in the form of cross-bracing, they only need to resist tension. When one brace is in tension, compression is induced in the other. 2.1.1.2 Diagonal Bracing In diagonal only a single diagonal member is used. This member is designed to resist both, tension as well as compression caused by lateral forces acting in both directions on the frame. Single bays of diagonal braces (Fig. 1(b) and (c)) respond differently according to the direction of loading. Configuration (b) may be much weaker and flexible in the direction causing compression in the braces, while configuration (c) will be weaker and more flexible in the storeys with compression braces, leading to the possibility of 220 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
soft-storey formation. 2.1.1.3 V Bracing The V-braced arrangements of Fig. 1(d) and (e) suffer from the fact that the buckling capacity of the compression brace is likely to be significantly less than the tension yield capacity of the tension brace. Thus there is inevitably an out-of balance load on the horizontal beam when the braces reach their capacity, which must be resisted in bending of the horizontal member. This restricts the amount of yielding that the braces can develop, and hence the overall ductility. 2.1.1.4 K Bracing The same out-of-balance force applies to K-braces (Fig. 1(f)) when the braces reach their capacity, but this time it is a much more dangerous horizontal force applied to a column dangerous because column failure can trigger a general collapse. For this reason, K-braces are not permitted in seismic regions by the code because of the inelastic deformation and buckling of K bracing member may produce lateral deflection of connected columns, causing collapse of structure. The K bracing and diagonal bracings is not considered in this study due to its instability for given load conditions and geometry. Fig 1: Examples of bracing schemes for concentrically braced frames: (a) X braced (b) diagonally braced (c) alternative diagonally braced (d) V braced (e) Inverted V braced and (f) K braced 2.1.2 Eccentrically Braced frames (EBFS) In Eccentrically Braced Frames (EBFs), some of the bracing members are arranged so that their ends do not meet concentrically on a main member, but are separated to meet eccentrically. The eccentric link element between the ends of the braces is designed as a weak but ductile link which yields before any of the other frame members. It therefore provides a dependable source of ductility and, by using capacity design principles, it can prevent the shear in the structure from reaching the level at which buckling occurs in any of the members. Arrangements such as (a) and (b) in Fig 2 also have architectural advantages in allowing more space for circulation between bracing members than their concentrically braced equivalent. An alternative arrangement with similar characteristics is the knee-braced frame. 2.1.2.1 Knee-braced frame The yielding element here is the knee brace, which remains elastic and stiff during moderate earthquakes, but yields to provide ductility and protection from buckling in extreme events. Unlike the link in the EBF, the knee brace does not form part of the main structural frame, and could be removed and replaced if it is damaged in an earthquake. 221 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
Fig 2: Examples of bracing schemes for eccentrically braced frames: (a) Knee braced ;(b) eccentric diagonal braced; (c) V braced. 3. Problem Modeling: Table: 1 Parameter Un-braced Structural System Braced Structural System 1. Geometry Details Plan Dimension 30X12 m Height of Structures 35 m Number of bays in X direction 5 Number of bays in Z direction 2 Width of Bay in X direction 6 m Width of Bay in Z direction 6 m Height of each Story 3.5 m Type of connection in Main Beam and column in Moment Connection Shear Connection X Direction Type of connection in Main Beam and column in Moment Connection Shear Connection Z Direction Support Conditions Fixed Support Pinned support Bracing Not Applicable Various Types 2. Section Properties Used Column UC sections from British code Beam UB sections from British code Bracings Not applicable UC sections 3. Primary Load cases Dead Load a. Self Weight Applied in Y-1 direction b. Cladding load 10 kn/m UDL all over periphery c. Floor Load of deck slab 5 kn/m 2 assuming 200 mm thick deck slab Live Load a. Floor Load for office building 2.5 kn/m 2 Earthquake Parameters a. Earthquake Code IS 1893: 2002 b. Seismic Zone III c. Importance Factor 1 d. Response Reduction 5 4 e. Soil Parameter Hard Soil f. Damping ratio 2 % for steel structure Wind Load Parameters a. Basic Wind Speed V b = 39 m/s For Pune region. b. k l = For category 3 Class one 1 c. k 2 = terrain, height and structure size factor For category 3 Class one varies as per height increases. d. k 3= topography factor 1 222 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
Fig.3.1: Inverted V bracing. Fig.3.2: V bracing. Fig.3.3: X bracing. Fig.3.4 Knee bracing Fig.3.5 Moment resisting frame Fig.3.6 Plan View (For All) Fig: 3 STAAD Pro. Models 223 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
4. Result and discussions: The results are discussed as per the analysis based on the loads and their combinations as per IS: 800 2007 The models used for the STAAD Pro. analysis are as showed in Fig:3 Graph 1 : Resultant storey displacement. Graph 2 : Storey displacement in X direction Graph 3 : Storey displacement in Z direction 224 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
The resultant storey displacement of the inverted V braced frame is less with compare to the all other bracing frames. From the above results it is all the displacements are within the limits TABLE 2: Resultant Storey displacement in mm. Bracing Storey Resultant Storey displacement in mm. X brace V Inv V Knee Moment Res Storey 1 4.522 3.248 4.290 9.115 5.817 Storey 2 9.870 7.552 8.835 18.869 13.440 Storey 3 15.972 12.741 14.006 29.213 21.079 Storey 4 22.611 18.928 20.039 39.883 29.226 Storey 5 30.033 26.116 26.485 50.648 37.005 Storey 6 37.740 33.499 33.218 61.079 44.220 Storey 7 45.596 41.441 40.086 70.775 50.742 Storey 8 53.614 49.279 47.089 79.246 58.715 Storey 9 61.350 56.824 53.870 85.893 64.718 Storey 10 68.643 63.916 60.200 90.393 68.311 TABLE 3: Storey displacement in X and Z Bracing Storey Storey displacement in X. (mm) X V Inv V Knee Moment Res Storey displacement in Z. (mm) X V Inv V Knee Moment Res Storey 1 3.014 1.048 1.902 8.301 5.213 0.680 0.627 0.285 4.013 5.281 Storey 2 6.936 3.652 4.924 17.489 12.580 2.535 2.753 2.043 10.487 12.724 Storey 3 12.011 8.593 9.052 27.472 19.995 5.320 6.587 4.673 17.659 19.894 Storey 4 18.084 14.400 14.138 37.918 27.886 8.967 10.667 8.228 24.734 26.746 Storey 5 24.972 20.938 20.105 48.460 35.492 13.503 15.029 12.557 31.447 33.558 Storey 6 32.464 28.015 26.682 58.773 42,572 18.755 19.548 17.387 37.871 40.101 Storey 7 40.325 35.505 33.595 68.423 49.059 24.527 24.402 22.564 43.889 45.623 Storey 8 48.309 43.149 40.590 76.874 56.951 30.632 29.432 27.953 49.313 50.447 Storey 9 56.168 50.666 47.477 83.504 62.934 36.892 34.475 33.420 54.031 53.599 Storey 10 63.689 57.857 53.960 85.045 66.617 43.152 39.396 38.799 58.255 55.360 From the above observation, it is clear that the displacement in X direction is maximum for the Knee bracing and minimum for the Inverted V bracing and the displacement in Z direction is maximum for the Knee braced frame and minimum for Inverted V and V braced frame 225 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
Graph 4 : Graph showing total weight of structure in kn. TABLE 6.5 : Total weight of structurei n kn Sr.No Type of bracing Total Weight 1 X bracing 1984.741 2 V bracing 1957.173 3 Inverted V 1974.065 4 Knee Bracing 1971.584 5 Moment Resisting 1976.007 As shown in above graph 4 the total weight of the members in the V braced frame is less. The X braced frame have more weight compare to other structure. 5. Conclusion: After carrying out detailed study by using STAAD Pro software on different bracing systems, for high-rise structures on parameters like weight, deflection and suitability for providing openings following conclusions are drawn. 1. By comparing the results (Graph no.2 and 3) it is to conclude that the lateral deflection for the concentric braced frames (X braced, V braced and Inverted V braced) is minimum than that of Knee (eccentric) braced and moment resisting frame. 2. By comparing the results (Graph no 1, 2, 3) can be concluded that resultant and X,Y displacement of Inverted V bracing is less than that of other braced structures. 3. By comparing results (Graph 4) it can be concluded that for the given structure minimum member weight is attained for V braced frame. (V braced frame weight is 2% less than that of X braced frame, 1% less than Inverted V braced frame,1% less than Knee braced frame.) 4. Steel bracings reduce flexural and shear demands on beams and columns, and transfer the lateral loads through axial load mechanism. This results into lighter weight sections for columns. (In present design more heaviest column sections used for moment resisting frame is U.C 356x368x153) 226 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed
5. From above it can be concluded that V bracing is the most efficient bracing in minimizing weight and displacement as compare to other types of bracings. However, this bracing obstructs openings; at such situation Inverted V brace should be used. References: [1] A Kadid, D.Yahiaoui Seismic Assessment of Braced RC Frames. 2011 Published by Elsevier Ltd, A. Kadid and D. Yahiaoui / Procedia Engineering 14 (2011),Pages 2899 2905 [2] Behruz Bagheri Azar, Mohammad Reza Bagerzadeh Karimi Study the Effect of using Different Kind of Bracing System in Tall Steel Structures American Journal of Scientific Research, ISSN 1450-223X Issue 53 (2012),Pages 24-34 [3] K.K.Sangle, K.M.Bajoria, V.Mhalungkar. Seismic Analysis of High Rise Steel Frame Building With And Without Bracing [4] Mahmoud Miri, Soleiman Maramaee The effects of asymmetric bracing on steel structures under seismic loads. World Academy of Science, Engineering and Technology 50 2009, Pages 939-943 [5] Manish Takey, S.S Vidhale Seismic response of steel building with linear bracing system International Journal of Electronics, Communication & Soft Computing Science and Engineering I(SSN: 2277-9477), Volume 2, Issue 1, Pages 17-25 [6] Mina Naeemi and Majid Bozorg Seismic Performance of Knee Braced Frames. World Academy of Science, Engineering and Technology 50 2009, Pages 976-980 [7] S. Maramaee, The Effects of Place and Kind of Bracing on Steel Structures. Middle-East Journal of Scientific Research 11 (5): 674-678, 2012. [8] Viswanath K.G, Prakash K.B, Anant Desai. Seismic Analysis of Steel Braced Reinforced Concrete Frames International Journal Of Civil And Structural Engineering. Volume 1, No 1, 2010 (ISSN 0976 4399)Pages114-122 [9] Zasiah Tafheem, Shovona Khusru, Structural behavior of steel building with concentric and eccentric bracing: A comparative study. International Journal Of Civil And Structural Engineering Volume 4, No 1, 2013. ISSN 0976 4399, Page 12-19. [10] Indian Standard code I.S 800-2007. [11] Indian Standard code I.S 875 Part I to V 1987 [12] Indian Standard code IS 1893 : 2002 [13] American Institute of Steel Construction (AISC) Manual -Seismic Provisions for Structural Steel Buildings. [14] Handbook of Steel Structure published by Institute for steel development and growth. (INSDAG) 227 Y. U. Kulkarni, P. G. Chandak, M. K. Devtale, S.S. Sayyed